Various sorbents/ion exchange materials are available for metal/nitrate ion sequestration. Unfortunately, however, all of these suffer from the disadvantage that they possess at most two or three functional groups capable of ion interaction per attachment site. Additionally, these conventional materials are in bead (porous) form and thus, are not suited for effective utilization in convective flow applications.
As a specific example of this, ion-exchange resins (IERs), such as strong acid or weak acid cationic exchangers, have been used extensively to recover heavy metals and/or to prepare high quality water. The typical theoretical capacity of these IERs is five meq/gram (see "Ion-Exchange Resins and Related Polymeric Adsorbents", Technical Bulletin AL-142, Aldrich Chemical Company). This capacity is quite low. For example, if one considers a typical charged metal ion such as nickel (II) a maximum uptake of only 0.15 gram of metal per gram of IER is possible. Further, the requirement for the regeneration of these IERs is a serious disadvantage as it produces concentrated waste solutions. Still further, the use of ion exchange beads requires column operations with high pressure drops and the rate of metal ion uptake is thereby limited by diffusion control.
Of course, there are many industrial situations where it is required to convert metal ions from the solution state to a solid form. This is done in order to facilitate the disposal of such metal species. In still other situations subsequent regeneration is not a consideration and/or a liquid volume reduction and entrapment of low levels of radioactive ions in a solid form is required. In these instances and applications, IERs have a significant cost disadvantage.
It is known, however, that liquid volume reduction and metal ion entrapment may be achieved using inexpensive, commercially available, high molecular weight cut-off ultrafiltration or microfiltration membranes in which internal surface areas range from 50-200 m.sup.2 /gm. The most inexpensive materials used to prepare such membranes are cellulose and its derivatives, cellulose acetate and cellulose triacetate. Examples of such membranes are disclosed, for example, in U.S. Pat. Nos. 4,824,870 and 4,961,852 both to Pemawansa et al.
Both flat sheet and wide bore hollow fiber (200-300 .mu.m in diameter) configurations are readily available commercially. However, direct use of these membranes for adsorption of a metal ion such as nickel (II) assuming the size of 6 .ANG. for the hydrated metal ion species and an internal surface of 100 m.sup.2 /gm of membrane, yields a maximum surface entrapment capacity of 0.034 grams of nickel per gram of membrane. This, of course, is too low for efficient liquid volume reduction. In fact, where only single complexation sites are available, one will require a relatively high surface area of membrane (approximately 3000 m.sup.2 /gm) in order to achieve a 1 gram of nickel uptake per gram of membrane.
In U.S. Pat. No. 4,604,204 to Linder et al., a cellulose acetate containing membrane having pore sizes of preferably 10-500 angstroms is treated with reagents such as di-aldehydes and diisocyanates that react with the hydroxyl groups of the membrane. The reagents function as linker molecules for the attachment of a polyfunctional oligomer or polymer. These membranes are made to exclude metal ions from pores rather than to entrap metals inside the pores.
While effective in excluding metal ions, this approach has several shortcomings. First, it should be appreciated that the linker molecules fill some space and tend to interfere with and close the relatively small diameter pores to subsequent reaction. Second, it should be appreciated that many times both functional groups of the linker molecules react with hydroxyl groups of the membrane leaving none available to subsequently react with the polyfunctional oligomer or polymer. Thus, the number of available sites for polyfunctional oligomer or polymer attachment is, in fact, quite limited thereby limiting the effectiveness of the modified membrane.
Still further, any cellulose based membrane suffers from an unacceptable degree of acid and/or solvent instability which limits or prevents its use in many applications. Thus, no form of chemical modification makes their use acceptable in these environs.
A need is therefore identified for an improved chemically activated microfiltration membrane that may be utilized for heavy metal ion sequestration and other purposes (e.g. nitrate ion sequestration) and that is characterized by a relatively high entrapment capacity heretofore unavailable in the art.